The detrimental impact of alcohol on highway safety has been well documented. Marijuana's opponents claim that it, too, causes significant impairment and that any increase in use will lead to increased highway accidents and fatalities.

THE FACTS

In high doses, marijuana probably produces driving impairment in most people. However, there is no evidence that marijuana, in current consumption patterns, contributes substantially to the rate of vehicular accidents in America.

A number of studies have looked for evidence of drugs in the blood or urine of drivers involved in fatal crashes. All have found alcohol present in 50% or more. Marijuana has been found much less often. Furthermore, in the majority of cases where marijuana has been detected, alcohol has been detected as well. 77

For example, a recent study sponsored by the U.S. National Highway Traffic Safety Administration (NHTSA) involving analysis of nearly 2000 fatal accident cases, found 6.7 % of drivers positive for marijuana. In more than two-thirds of those, alcohol was present and may have been the primary contributor to the fatal outcome. 78

To accurately assess marijuana's contribution to fatal crashes, the positive rate among deceased drivers would have to be compared to the positive rate from a random sample of drivers not involved in fatal accidents. Since the rate of past-month marijuana use for Americans above the legal driving age is about 12 percent, on any given day a substantial proportion of all drivers would test positive, particularly since marijuana s metabolites remain in blood and urine long after its psychoactive effects are finished.

A recent study found that one-third of those stopped for "reckless driving" between the hours of 7 p.m. and 2 a.m. - mostly young males - tested positive for marijuana (and no other drugs).79 To be meaningful, these test results would have to be compared to those from a matched control group of drivers.

A number of driving simulator studies have shown that marijuana does not produce the kind of psycho-motor impairment evident with modest doses of alcohol. 80 In fact, in a recent NHTSA study, the only statistically significant outcome associated with marijuana was that drivers drove more slowly. 81

A recent study of actual driving ability under the influence of cannabis - employing the same protocol used to test the impairment-potential of medicinal drugs - evaluated the impact of placebo and three active THC doses in three driving trials, including one in high-density urban traffic.

Dose-related impairment was observed in drivers' ability to maintain steady lateral position. However, even with the highest dose of THC, impairment was relatively minor - comparable to that with blood-alcohol concentrations of between .03 and .07 % and many legal medications. Drivers under the influence of marijuana also tended to decrease their speed and approach other cars more cautiously.

While recognizing some limitations of this study, the authors conclude that "THC is not a profoundly impairing drug." 82